Optical fibers that guide light by total internal reflection consist of a cylindrical core that has a higher refractive index than the surrounding cladding. For single-mode operation, the core size and the index difference between the core and cladding are such that only the fundamental mode is propagated for a given spectral bandwidth, as determined by the characteristic waveguide number or V-number. (A single mode fiber is an optical fiber that is designed for the transmission of a single ray or mode of light).
With reference to FIG. 1, an example of a single-mode optical fiber system is indicated at 10. The system 10 comprises a single-mode optical fiber 12 having an internal core 16 within external cladding 18. The core 16 and cladding 18 are protected within an external buffer 14, which is shown as being stripped along the length of the fiber that is to be processed. A cross-section of the fiber 12 across line A-A is indicated at 20. Typical dimensions for a standard telecommunications optical fiber 12 would be 9 μm diameter for the core 16 and 125 μm diameter for the cladding 18.
In the case of high-power transmission through standard single-mode fibers, end terminations of fibers and in-line splices or interconnects can introduce undesirable back-reflections and facet distortions that can lead to system damage and failure. Further complications can arise due to, e.g. dirt at the end termination and/or between the end of the fiber and an associated connector. In order to reduce this problem, it is desirable to reduce the power density by expanding the mode field diameter. This can be achieved using a variety of techniques, including fiber tapering, thermal core diffusion, lensing including bulk and grin lensing, fiber end shaping, and splicing on dissimilar fibers including e.g. multimode fibers. However, where a typical beam diameter of ˜50 μm is desired, each of these solutions has associated problems.
In the case of tapering, the fiber becomes small, difficult to handle and more sensitive to external influences—making it difficult to package. The diffusion approach is limited in the extent to which the beam may be expanded before loss becomes significant. Lensing does not reduce the optical power density at the fiber end-face, generally involves the introduction of free-space facets, back-reflections, glues, alignment issues and loss within in-line fiber pigtailed bulk-optic sub-systems, and is expensive. Using dissimilar fibers requires a splice and introduces back-reflections and loss where the beam diameter is not mode-matched, and it can be a relatively expensive process compared with the approach described here.
An alternative technique which has been recently proposed is that of fiber fattening (also referred to as fiber up-tapering or fiber dilation), discussions of which may be found in [1] PhD thesis, Elaine M. O'Brien, Lightwave Technology Research Centre, University of Limerick; [2] “Up-tapering of optical fibers using a conventional flame tapering rig”, G. Kakarantzas, L. Prill-Sempere and P. St. J. Russell, CFK2, Optical Society of America-CLEO/QELS Conference, 2007; and [3] “Adiabatic dialated standard and speciality optical fibers”, N. Healy, D. F. Murphy, E. M. O'Brien and C. D. Hussey, Poster080 Photonics Ireland 2007 (Galway), which are incorporated herein by reference in their entireties.
In known fiber fattening processes, a fiber to be fattened is positioned between a pair of holders, and a heat source is applied along a length of the fiber to soften the core and cladding material. The heat source may be a conventional flame, or could comprise an arc, laser, or other heat source. The action of heating a fiber that is subjected to a compressive force above its glass transition temperature results in the expansion of the width of the fiber in conjunction with a reduction of the fiber length.
An example of the effects of up-tapering is shown in FIG. 2, which shows fiber 12 after up-tapering has taken place. As can be seen from FIG. 2, the length of the fiber 12 has decreased, the newly-fattened fiber 12a now showing transitions 22 between the end portions of the fiber 12a and the expanded middle portion 100 of the fiber 12a. An indication of the cross-section of the fiber 12a along line B-B is indicated at 24. Typical dimensions of the expanded cross-section after up-tapering would be 30 μm diameter for the core 16 and 375 μm diameter for the cladding 18.
Such up-tapered fibers provide for numerous advantages, e.g. the reduction of optical power density, the improvement of mode-matching between spliced dissimilar fibers, and the flattening of the wavelength response of fused directional fiber couples.
The up-tapering process is limited by a number of conditions which must be satisfied:                1. The adiabatic condition needs to be satisfied—i.e. the transition between the fattened and non-fattened sections needs to be sufficiently smooth to ensure the launch of only the local fundamental mode, so as to avoid any losses due to the transition. For the transitions to be adiabatic, at any point along the processed fiber, the transition must satisfy the slowness criterion:        
                        ⅆ        a                    ⅆ        z                  ⪡      a          z      b                       This is known as the adiabatic condition, wherein a is the core radius at any position z along the transition such that da/dz defines the taper angle and zb is the beat length or period of power oscillations between the excited modes of the system. The shortest beat length can be considered as that between the HE11 mode (i.e. the designation for the fundamental mode of an optical fiber) and the closest mode of the same symmetry, the HE12 mode. Transition losses due to non-compliance with the adiabatic condition are one of the more considerable limitations in fiber up-tapering.        2. A waveguide needs to be maintained. In conventional optical fibers, light is guided by total internal reflection, which is made possible by the index difference between the core and cladding. In general, the cladding used is silica, and the core has a raised index that is achieved by doping silica with germanium. The heating of the fiber during the fattening process results in thermal diffusion of the core dopant, germanium. With diffusion, the index difference between the cladding and core is reduced and the waveguide becomes weaker. Unless the diffusion is controlled, the diffusion may occur to such a degree that there will effectively no longer be an index step, and the optical fiber no longer acts as a waveguide. Further, any diffusion that does occur needs to satisfy the adiabatic condition given in 1. above.        3. Physical size mismatch. As a fiber is fattened, the fattened section becomes larger and heavier to the point that the standard fiber leads are no longer able to support its weight, and an inevitable sagging will take place.        4. Mode-area limit. Taking the example of large mode area fibers for lasing, as the mode area is increased, the fiber's ability to maintain single-mode only propagation is reduced, and light couples into the other modes of the fat, and accordingly highly multimode, structure. A number of techniques can be used to strip out higher mode behaviour and therefore maintain single-mode operation. This is a minimal concern in cases where the fattened fiber section is fattened over a short length.        
Accordingly, current fiber fattening techniques are limited to the expansion that can be achieved, typically up to ˜2.25 times dilation of the original fiber. It is an object of the invention to provide a new method of fiber fattening method that allows for greater dilation of fibers, while satisfying the limitations described above.